About the BritNed Interconnector
The BritNed interconnector is a high voltage direct current (HDVC) bipolar cable linking the UK and the Netherlands, the cable is owned and operated the BritNed Development Limited. The BritNed interconnector is part of a joint venture by National Grid and TenneT to ensure continuing security and diversity of supply, helping to facilitate low carbon generation.
The interconnector comprises of two bundled cables with a combined power rating of 1000M over the 250km subsea section. The cables were produced and installed by ABB in seven sections from 2009 to 2010 and went into service in April 2011.
2017 Marine Survey
In April 2017, Bibby HydroMap Ltd. were commissioned by BritNed Development Limited to perform a marine geophysical and depth of burial survey of the BritNed Interconnector between The UK and The Netherlands.
The main objectives of the survey were as follows:
- To confirm the position of the cable in the x, y and z;
- To measure and document the depth of burial of the cable, both in terms of absolute levels and adjacent seabed levels;
- To document any areas of scour, particularly in the vicinity of third party crossings;
- To locate any debris within 20m of the cable position.
The BritNed cable was to remain energised throughout survey operations therefore traditional methods of measuring depth of burial using techniques that require the application of a tone to the cable were not possible. With the interconnector being a HVDC cable there was also negligible tone emanating from the cable itself.
Despite the challenging tidal currents along the cable route, both the swath bathymetric survey and the mapping of seabed features did not present a significant challenge for Bibby HydroMap in terms of survey design. The main obstacle came in the form of how to accurately map the depth of burial of the HVDC cable as it was to remain in service throughout survey operations. More traditional methods of cable depth of burial measurement include a requirement to either interrogate the tone of a tone carrying cable or to induce a tone into the cable. Since the BritNed cable is a HVDC cable, it presents a negligible tone of its own and as the cable is to remain energised, it was also impossible to apply any external tone to the cable.
To accurately map bathymetry, seabed features and depth of burial along the interconnector Bibby HydroMap employed a two-phase acquisition strategy. The first phase was to accurately map bathymetry and seabed features which saw Bibby Athena mobilised with a hull mounted, dual head, full rate, Reson 7125 MBES system and a dual frequency Edgetech 4200 side scan sonar, whilst Proteus was mobilised with a hull mounted Reson T-20 multibeam echo sounder and Edgetech 4200 side scan sonar.
During the second phase of acquisition the Pangeo Sub-Bottom Imager (SBI) was employed to accurately map the depth of burial of the interconnector. For the offshore section, the d’ROP, deployed from the MV Bibby Athena, allowed for the acquisition of depth of burial data using the SBI at a constant optimal altitude of 3.5m above the seabed and with negligible vehicle pitch, roll or heave motion. The d’ROP was also able to maintain its position along the length of the route in order to be able to image the cable.
For the inshore section, the Pangeo SBI was deployed from Proteus. This was to be performed in two separate towing configurations; one subsea towing configuration to be used in water depths of between 10m - 3.5m LAT and one surface towed, floating configuration to be used in water depths of 3.5m or lower.
Offshore Depth of Burial Operations
During the offshore depth of burial operations, the d’ROP was operated with the Pangeo SBI and the NORBIT iWBMS dual head multibeam system.
Acquisition with the d’ROP/SBI combination was typically performed at speeds of around 1 to 1.5 knots. This gave a good speed of coverage, allowed the vessel and d’ROP to maintain course whilst allowing the cable to be monitored real-time. Environmental conditions begun to compromise d’ROP acquisition when tidal flow exceeded 3 knots. Deployment and recovery of the d’ROP was typically not performed in wave heights of more than 1.3m and line keeping became challenging in wind velocities of over 20 knots.
The requirement to acquire a high-resolution bathymetry dataset along the interconnector route was achieved by utilising the NORBIT iWBMS dual head multibeam system mobilised onto the d’ROP along with the SBI. This provided a high-resolution data set centered over the cable route but reduced coverage to a +/-12.5m corridor.
Inshore Depth of Burial Operations
Following completion of the offshore depth of burial operations, the SBI was demobilised from the d’ROP and transferred to Proteus. The SBI was converted to the shallow water assembly used for towing and floating in shallow water. In water depths of between 10 and 3.5 m this would be performed by towing the SBI subsea, the optimum flying height was maintained throughout survey operations by manual winch operations. In water depths of less than 3.5m the SBI was towed with floatation buoys were attached to the SBI for the shallow water depths to enable the SBI to be surface towed. The unit was towed to ensure the source and receiver were just below the water surface.
Depth of burial operations were successfully carried out in the inshore sections over distances of 2km at the UK sector and 1km over the Dutch sector.